1. Field on the Invention
The present invention relates to a process for manufacturing high-quality wafers, both with and without devices, that can be used in the semiconductor industry. In particular, the present invention relates to a technique for manufacturing wafers having at least one thin layer of semiconductor material, typically silicon, obtained by detachment or separation of a bulk portion due to the introduction of an exfoliating agent in a starting substrate having a thickness greater than the desired one.
2. Discussion of the Related Art
This detachment technique enables so-called silicon-on-insulator (SOI) substrates to be obtained and is described in numerous documents.
For example, U.S. Pat. No. 5,374,564 to Bruel describes a process comprising implanting hydrogen ions on the front side of a silicon wafer; bonding the implanted silicon wafer to a support wafer having a surface oxide layer, so that the surface oxide layer bonds to the front side of the silicon wafer; and annealing at a temperature higher than 500° C. In this way, the implantation of hydrogen ions causes a layer of gas microbubbles to form within the silicon wafer at a depth equal to the mean penetration depth of the hydrogen. During the final thermal treatment, the microbubble layer determines the splitting between the overlying layer, which forms a thin silicon layer bonded to the support wafer, and the rest of the first wafer.
U.S. Pat. No. 6,013,567 to Henley and U.S. Pat. No. 6,387,829 to Usenko et al., describe processes like Bruel process referred to above. They differ only in the detachment technique. As indicated, Bruel uses a thermal treatment, Henley uses a jet of a pressurized fluid aimed against the edge of the wafer, and Usenko et al. use different sources of energy, such as ultrasound, hydrostatic pressure, hydrodynamic pressure, infrared light, or mechanical force.
The main disadvantage of the described known processes lies in the high dose of implanted hydrogen atoms, of the order of 1016-1017 atoms/cm2, necessary for creating the microbubble layer. In fact, the hydrogen distributes approximately as a Gaussian, the extension whereof is determined by the longitudinal distribution of the process of ion-silicon interaction. Outside the peak region, the dose is sufficiently high to induce the formation of small bubbles and of defects <111> in the monocrystalline silicon. Consequently, the thin silicon layer overlying the implanted layer is defective, and thus leads to the formation of a SOI substrate of poorer quality than the monocrystalline silicon currently used.
To overcome this problem, the possibility of reducing the dose of hydrogen by a factor of 5-10 has been studied by implanting, in the same region, hydrogen and helium. Tests carried out have, however, highlighted that also this solution does not enable a thin layer completely devoid of defects to be obtained.
The possibility of introducing hydrogen via a plasma has also been explored. The hydrogen is gettered via a buried trap layer. The trap layer can be created by implanting and subsequent annealing a P type dopant, such as boron, acting as gettering material, see, for example, U.S. Pat. No. 6,346,458 to Bower.
Tests have demonstrated, however, that the use of a plasma with high-energy particles produces defects on the surface of the specimen, and hence it is not possible to obtain the quality necessary for integration of components.
Finally, U.S. Pat. No. 6,696,352 B1 to Carr et al. describes a process comprising implanting silicon ions (at least 1013 atoms/cm2) in a first wafer so as to form a trap layer formed by defects localized principally at the ion end-of-range; application, on a different wafer, of an adhesive layer capable of releasing hydrogen ions after polymerization; bonding the two wafers; polymerizing the adhesive layer so as to obtain release and diffusion of the hydrogen atoms in the first wafer; gettering the hydrogen atoms at the trap layer; forming microbubbles; and detaching a portion of the first wafer.
It is to be noted that in the foregoing sequence, as hereinafter, the term “ion end-of-range (EOR)” indicates a region of the specimen, parallel to its surface, where the implanted ions are localized. This region also houses an accumulation of silicon interstitials that create EOR defects. The distance of this region from the surface of the specimen depends upon the type of ion, its energy, and the target. For a same energy, light ions (e.g., B) have a greater EOR than heavy ions (e.g., As), while, for the same ions, the increase of energy causes an increase in the distance between the EOR and the surface.
Also this process suffers from the above problems, due to the hydrogen atoms crossing the thin layer of the first wafer and to the interaction of the hydrogen with the defects created by the silicon ions throughout the layer.
The aim of the invention is thus to provide a process for manufacturing a thin high-quality layer of semiconductor material, such as silicon, so as to enable integration of electronic components.